This invention relates generally to implantable defibrillators and more particularly to a method and apparatus for providing more efficient ventricular defibrillation shocks.
Cardiac arrhythmias can generally be thought of as disturbances of the normal rhythm of the heart muscle. Cardiac arrhythmias are broadly divided into two major categories, bradyarrhythmia and tachyarrhythmia. Tachyarrhythmia can be broadly defined as an abnormally rapid heart (e.g., over 100 beats/minute, at rest), and bradyarrhythmia can be broadly defined as an abnormally slow heart (e.g., less than 50 beats/minute). Tachyarrhythmias are further subdivided into two major sub-categories, namely, tachycardia and fibrillation. Tachycardia is a condition in which the electrical activity and rhythms of the heart are rapid, but organized. Fibrillation is a condition in which the electrical activity and rhythm of the heart are rapid, chaotic, and disorganized. Tachycardia and fibrillation are further classified according to their location within the heart, namely, either atrial or ventricular. In general, atrial arrhythmias are non-life threatening, chronic conditions, because the atria (upper chambers of the heart) are only responsible for aiding the movement of blood into the ventricles (lower chambers of the heart), whereas ventricular arrhythmias are life-threatening, acute events, because the heart""s ability to pump blood to the rest of the body is impaired if the ventricles become arrhythmic. This invention is particularly concerned with treatment of ventricular fibrillation.
Since an individual who experiences fibrillation typically will not always be immediately accessible by emergency care technicians and their equipment, and/or will become incapacitated and unable to beckon such care, implantable cardiac stimulation devices have become critical delivery systems of emergency care for many patients with chronic heart failure problems.
Various types of implantable cardiac stimulation devices are presently available and used for delivering various types of cardiac stimulation therapy in the treatment of cardiac arrhythmias. The two most common types which are in widespread use are pacemakers and implantable cardioverter defibrillators (ICDs).
Pacemakers generally produce relatively low voltage pacing pulses which are delivered to the patient""s heart through low voltage, bipolar pacing leads, generally across spaced apart ring and tip electrodes thereof which are of opposite polarity. These pacing pulses assist the natural pacing function of the heart in order to prevent bradycardia.
On the other hand, ICDs are sophisticated medical devices which are surgically implanted (abdominally or pectorally) in a patient to monitor the cardiac activity of the patient""s heart, and to deliver electrical stimulation as required to correct cardiac arrhythmias which occur due to disturbances in the normal pattern of electrical conduction within the heart muscle. In general, an ICD continuously monitors the heart activity of the patient in whom the device is implanted by analyzing electrical signals, known as electrograms (EGMs), detected by sensing electrodes positioned in the patient""s heart. More particularly, contemporary ICDs include waveform digitization circuitry which digitizes the analog EGM produced by the sensing electrodes, and a microprocessor and associated peripheral integrated circuits (ICs) which analyze the digitized EGM in accordance with a diagnostic algorithm implemented by software stored in the microprocessor. Contemporary ICDs are generally capable of diagnosing the various types of cardiac arrhythmias discussed above, and then delivering the appropriate electrical stimulation/therapy to the patient""s heart, in accordance with a therapy delivery algorithm also implemented in software stored in the microprocessor, to thereby correct or terminate the diagnosed arrhythmias. Typical electrical stimulus delivery means used in ICDs involve an energy storage device, e.g., a capacitor, connected to a shock delivering electrode or electrodes. Contemporary ICDs are capable of delivering various types or levels of electrical therapy. U.S. Pat. No. 5,545,189 provides a representative background discussion of these and other details of conventional ICDs, and the disclosure of this patent is herein incorporated by reference.
One conventional method of electrical shock therapy for treating ventricular arrhythmia is to deliver a single burst of a relatively large amount of electrical current through the fibrillating heart of a patient by an ICD supported-electrode configuration installed in or about the patient""s heart. For a given ventricular fibrillation episode, the minimum amount of energy required to defibrillate a patient""s ventricle is known as the ventricular defibrillation threshold (VDFT). However, in the treatment of an acute cardiac condition, such as ventricular fibrillation, conventional ICD-based therapies have encountered a dilemma in that while higher strength defibrillation shocks generally have a higher probability of success of achieving defibrillation than lower strength shocks, the countervailing consideration is that higher energy shocks demand commensurately greater ICD equipment capabilities and cost, such as in terms of batteries, capacitors, and so forth.
It has been experimentally observed that the likelihood of successful defibrillation has been shown to follow a sigmoidal shaped curve in which higher strength shocks have a higher probability of success than lower strength shocks. See, e.g., Davy J., et al., xe2x80x9cThe relationship between successful defibrillation and delivered energy in open-chest dogs: reappraisal of the xe2x80x9cdefibrillation thresholdxe2x80x9d concept,xe2x80x9d Am Heart J. 1987; 113:77-84. When a number of shocks are applied at the V50 level, 50% of applied shocks are expected to result in successful defibrillation. In order to interpret the increased probability of success in terms of percentage improvement in DFT, some previously published data is available to illuminate the issue. For a superior vena cava (SVC) lead and right ventricle (RV) lead configuration, for example, the probability of success curves have been developed to determine that (V80xe2x88x92V50)/V50=0.14. E.g., see Souza et al., xe2x80x9cComparison of upper limit of vulnerability and defibrillation probability of success curves using a nonthoracotomy lead system,xe2x80x9d Circulation, 1995, 91:1247-1252. By linear approximation of the central portion of the sigmoidal curve, this yields (V70xe2x88x92V50)/V50=0.09. This equation indicates that if the probability of success in achieving defibrillation at a certain voltage is 50%, then increasing the voltage by 9% will increase the probability of success to 70%.
Yet, the ICD device preferably should be designed to be as small in dimensions and light in mass as possible so as to be less cumbersome and bulky to the patient, so it generally will not be practical to significantly scale-up the power and voltage capabilities of an ICD device in many cases as the mode of increasing the probability of success in the delivery of defibrillation therapy.
Instead, it would be desirable to find ways to lower the VDFT for a given ICD size and power. Furthermore, a patient having an installed ICD may experience several or more acute separate fibrillation episodes a year requiring intervention by the installed ICD unit. Thus, it can be appreciated how lowering of the energy requirements demanded of the ICD would be desirable so as to prevent premature depletion of the batteries, and thereby increase the service life of the ICD device.
Also, while a patient experiencing a ventricular fibrillation episode, may or may not be conscious or semi-conscious, it is still possible that the patient could potentially perceive any programmed electrical stimulation treatment being performed on his/her heart during the episode. Thus, to mitigate any possible further trauma to the patient on account of any negative perceptions of the electrical jolts accompanying the VDF shocks, or, alternatively, to reduce the risk of inadvertent myocardial tissue damage from the delivered shock, it also would be desirable to reduce the ventricular defibrillation threshold (VDFT) for these additional reasons.
Thus, it is desirable for reasons of both increased device longevity and patient comfort/safety to reduce the amount of energy required to defibrillate a patient""s heart when using an implantable cardioverter defibrillator (ICD). However, this goal had not previously been fully satisfied in the ICD field despite active interest and numerous experimental studies reported in the relevant field.
As generally known, during ventricular fibrillation (VF), it has been observed that shocks of the same voltage can at some times achieve successful defibrillation (DF), yet fail at other times. It has been observed that a factor contributing to this phenomena is that the electrophysiological state of cardiac tissue in the so-called xe2x80x9clow gradient regionxe2x80x9d of the heart can exist in various conditions of depolarization and repolarization at various times.
The xe2x80x9clow potential gradient region,xe2x80x9d or xe2x80x9clow gradient regionxe2x80x9d for short, is that region of the heart tissue that is most remote from the defibrillation electrodes and thus experiences a lower electrical gradient relative to other portions of the heart at the time of delivery of a defibrillation shock. More specifically, the low potential gradient region thus is where the electric field lines generated by the current flowing between a pair of defibrillation electrodes positioned in the heart are the least densely spaced. The location of this region can vary to the extent that the potential gradients generated by a defibrillation shock depend upon the particular lead configuration of the defibrillation electrodes in the heart, the tissue conductivities, and torso geometry. The low potential gradient region can be located by measurement or intuitively.
Previous cardiac mapping studies have demonstrated that, following a failed defibrillation shock, the earliest sites of propagation from which post-shock activation wavefronts originate tend to appear from regions where cells are just emerging from their effective refractory period immediately prior to the defibrillation shock. See, e.g., Chen. P., et al., xe2x80x9cComparison of activation during ventricular fibrillation and following unsuccessful defibrillation shocks in open-chest dogs,xe2x80x9d Circ Res. 1990;66:1544-1560; Zhou, X., et al., xe2x80x9cEpicardial mapping of ventricular defibrillation with monophasic and biphasic shocks in dogs,xe2x80x9d Circ Res. 1993;72:145-160; Walcott, G., et al., xe2x80x9cMechanisms of defibrillation for monophasic and biphasic waveforms,xe2x80x9d PACE. 1994;17:478-498.
Prior studies also have shown that these earliest sites of propagation are the regions where extracellular potential gradients are lower than a critical value. It follows from this that the shock strength in the low gradient region can be too low to cause any extension of refractoriness in the local tissue. Moreover, in the low gradient region, shocks can cause action potential stimulation only when they are applied very late in the repolarization phase. See, e.g., Swartz, J., et al., xe2x80x9cThe conditioning prepulse of biphasic defibrillator waveforms enhances refractoriness to fibrillation wavefronts,xe2x80x9d Circ Res. 1991;68:438-449; Dillon, S., et al., xe2x80x9cOptical recordings in the rabbit heart show that defibrillation strength shocks prolong the duration of depolarization and the refractory period,xe2x80x9d Circ Res. 1991;69:842-856; Dillon, S., et al., xe2x80x9cSynchronized repolarization after defibrillation shocksxe2x80x94A possible component of the defibrillation process demonstrated by optical recordings in rabbit heart,xe2x80x9d Circulation. 1992;85:1865-1878. These results have been interpreted to suggest that one possible factor contributing to the probabilistic character of defibrillation is that the state of repolarization of tissue in the low gradient region is different at different times.
Despite the improved understanding being developed in the field on the relationship of the electrophysiological characteristics of the low gradient region of the heart and efficacy of defibrillation therapy, there still exists a need for a modality of delivering cardiac therapy that improves the probability of success of a ventricular defibrillation shock while also reducing ventricular defibrillation thresholds (VDTs) to reduce energy demands placed upon an ICD device and to reduce the risk of pain, trauma or myocardial tissue damage to a patient undergoing defibrillation treatment.
The above and other objects, benefits and advantages are achieved by the present invention as described herein.
The present invention relates to treatment therapies and systems for ventricular arrhythmias which reduce ventricular defibrillation threshold (VDFT) energy requirements and/or increase the probability of a successful outcome when the defibrillation shock is delivered at a given energy level.
In one embodiment of the invention, VDFT energy requirements have been demonstrated to be dramatically reduced by delivering a pacing regimen to the low gradient region of the heart in conjunction with a timed-delivery of a defibrillation shock in synchronization to activation sensed in the low gradient region. More specifically, following the detection of fibrillation, one or several successive pacing pulse trains are applied to a pacing electrode placed in the low gradient region of the heart to capture the tissue. Once capture of a substantial extent of the tissue of the low gradient region is achieved via pacing, a high energy defibrillation shock is delivered in a timed manner. In one specific implementation of this embodiment, the location of the low gradient region is the left ventricular (LV) freewall, and a pacing rate of about 70-99%, and more preferably 80-95%, of the ventricular fibrillation cycle length (VFCL) sensed at the low gradient region is applied to achieve capture in the low gradient region. Then, a defibrillation shock is delivered at the end of the pacing train, preferably with a time interval between the last pacing pulse and the delivery of the defibrillation shock being a duration of time of either about 80-95% of the pacing rate or about 5-20% of the pacing rate. The timing of the delivery of the defibrillation shock preferably is based on information contained in electrogram signals acquired in real time from a sensing site at the low potential gradient during fibrillation.
One of the specific findings of the present invention is that the defibrillation shock has a greatly increased probability of success if a substantial majority of the tissue in the low gradient region is in the process of activation by fibrillatory wavefronts or is about to be depolarized. In the first-mentioned case, the depolarization caused by defibrillatory wavefronts is thought to add to the depolarization caused when the defibrillation shock is delivered, and in the latter case, the tissue around the electrode is thought to be at the end of its refractory period and will hence require a lower voltage gradient by the defibrillation shock to become depolarized. When pacino is used to achieve regional capture in the low gradient region, as described herein, the timing of the defibrillation shock will be caused to occur during either one of the above-mentioned electrophysiological periods by delivering the shock after the last pulse at an interval of about 80-95% of the pacing rate or about 5-20% of the pacing rate, respectively. This mode of tiered therapy effectively reduces the ventricular defibrillation threshold (VDFT) that otherwise would be applicable in the absence of the preliminary pacing tier of therapy.
As compared to VDFT without regional capture being provided via pacing prior to delivery of the defibrillation shock, the VDFT with regional capture provided via pacing in accordance with this embodiment of the invention is significantly lowered, which, in turn, significantly reduces the battery and energy requirements of an ICD or like device for defibrillation.
In an alternate mode of this invention, real time sensing in the low gradient region is used to generate ventricular electrogram (EGM) information from one or more sensing sites in the low gradient region, and when the sensed data indicates a substantial extent of tissue is simultaneously in the process of activation or is about to be depolarized, then the defibrillation shock is immediately delivered. This alternative mode of therapy involves xe2x80x9cpassive-timingxe2x80x9d in the sense that no intervention effort is made to disturb the natural electrophysiological state of the heart with external electrical stimuli until one of several opportunistic electrophysiological states are detected as occurring in the myocardial tissues of the low gradient region. Namely, the indication that a substantial extent of myocardial tissue is in the process of activation or is about to be depolarized in the low gradient region, and thus is more likely to respond favorably to a defibrillation shock, has been found to be highly predictable by the occurrence and detection of certain electrophysiological states in the low gradient region.
In one implementation, it has been found that the amount of reduction achieved in the DFT energy, or, alternatively, the probability of a successful outcome when the defibrillation shock is delivered at a given energy level, increases as a direct positive function of the percentage of the low gradient region that is in downstroke when the defibrillation shock is delivered. In implementing this embodiment, where one sensor is monitored in the low gradient region, then the defibrillation shock is delivered when the EGM is in the downstroke. Where two or more separate sensors are monitored in the low gradient region, an initial monitoring period of, for example, about 2-4 seconds, is conducted in which a defibrillation shock is delivered when and if all electrograms are simultaneously determined to be on downstroke. If the initial monitoring period elapses without that occurring, then the defibrillation shock is delivered the next time a majority ( greater than 50%), and more preferably  greater than 80%, of the EGMs from the sensed sites are simultaneously on downstroke. This technique involves a binary classification of the slope of the EGM profile being monitored in real time as being on an upstroke (i.e., where the slope of the EGM curve is numerically positive in value) or downstroke (i.e., where the slope of the EGM curve is numerically negative in value). This approach increases the amount of tissue depolarized at the end of the defibrillation shock and thereby enhances the probability of reducing the VDFT.
In another implementation, besides the above-mentioned binary classification of slope method for timing the delivery of the defibrillation shock, it is also possible to reference other EGM quantities, such as the amplitude of the electrogram and the magnitude of the slope of the electrogram sensed from the low gradient region, and use these parameters instead for timing the delivery of the defibrillation shock. Namely, another finding of the present invention is that either a relatively large amplitude or a large negative slope observed at an electrogram from the low gradient region has been found to indicate the presence of a large and rapidly moving activation wavefront over the low gradient region, which in turn indicates an increased probability of a large percentage of low gradient region tissue being on the downstroke of its EGM. It has been found that immediately delivering the defibrillation shock when the magnitude of the downstroke is sensed to be relatively large in amplitude or negative slope value results in a increased probability of success of defibrillation. Moreover, there exists an increased probability of a lower VDFT being exploitable at that time.
In further embodiments of the present invention, there are systems provided for implementing the various above-introduced methods of the invention.
For purposes of this application, the following terms have the indicated meanings:
Capture: means pacing of the ventricle from one or more sites where each pacing stimulus results in a repeatable activation pattern of the entire ventricle. The wavefronts originate at the pacing electrodes and the phase relationship between the pacing stimulus and the activation of each section of the ventricular tissue remains constant throughout the pacing event.
Entrainment: means the same as capture.
Regional capture: pacing of the ventricle from one or more sites where the stimulus results in wavefronts which depolarize only a portion of the myocardium surrounding the electrode or electrodes. The spatial extent of the depolarization caused by the pacing stimulus changes from beat to beat and occasionally may result in almost no propagated response. The wavefronts activating the captured region originate at the pacing electrode. The phase relationship remains constant between the pacing stimulus and activation of each section of myocardium within the region that is captured.
Phase-locking: pacing of the ventricle from one or more sites which results in wavefronts that appear to be constant in phase with the pacing stimulus but where there does not appear to be a cause and effect relationship. That is, the wavefronts do not appear to originate at the pacing sites and small changes in phase between the pacing stimulus and the activation of each section of a region occur over time. As a qualification, where EGM data on the ventricle is limited, it is often difficult to differentiate between phase-locking and capture, as defined herein, and, for those cases, phase-locking terminology is used herein to refer to both capture and phase-locking.
xe2x80x9cVentricular Defibrillation Thresholdxe2x80x9d or xe2x80x9cVDFTxe2x80x9d: The minimum amount of electrical energy required to defibrillate a fibrillating ventricle of a patient.
xe2x80x9cVentricular Fibrillation Cycle Lengthxe2x80x9d or xe2x80x9cVFCLxe2x80x9d, for short: the timing required between two consecutive depolarization wavefronts to traverse the same location is the ventricular fibrillation cycle length (VFCL).
xe2x80x9cPacing Ratexe2x80x9d: also referred to herein as the xe2x80x9cS1-S1xe2x80x9d interval, meaning the time intervals between delivery of successive pacing pulses.
xe2x80x9cCoupling Interval for Pacing Initiationxe2x80x9d or xe2x80x9cCIPIxe2x80x9d, for short: means the time delay between the last local activation sensed, as the trigger, and the start thereafter of the first pulse of the pacing train.
xe2x80x9cCoupling Interval for Defibrillation Shockxe2x80x9d or xe2x80x9cCIDSxe2x80x9d, for short: also referred to herein as the xe2x80x9cS1-S2xe2x80x9d interval, meaning the time interval between the last pulse of a pulse train and the specific time thereafter when a VDF shock, i.e., the defibrillation trigger, is delivered.
xe2x80x9cLow potential gradient region of ventricular tissuexe2x80x9d or xe2x80x9clow gradient regionxe2x80x9d for short: the region in the heart, as described supra, where the electric field lines generated by the current flowing between a pair of defibrillation electrodes positioned in the heart are the least densely spaced. The location of this region can vary to the extent that the potential gradients generated by a defibrillation shock depend upon the particular lead configuration of the defibrillation electrodes in the heart, the tissue conductivities, and torso geometry. The low potential gradient region can be located by measurement or intuitively.
xe2x80x9cUpstrokexe2x80x9d and xe2x80x9cdownstrokexe2x80x9d terminology herein relates to the electrogram morphology at each sensing electrode as being classified, respectively, as being on an xe2x80x9cupstrokexe2x80x9d when the slope of the electrogram (dV/dt) is greater than zero, and on a xe2x80x9cdownstrokexe2x80x9d when the slope of the electrogram (dV/dt) is less than zero. Further, the upstroke and downstroke terminology will be understood to be a function of the polarity of the connections to the sensing amplifier. In this regard and for purposes of simplifying the descriptions herein, the upstroke and downstroke terminology are premised on the sensing electrodes being connected to the positive side of the sensing amplifier and the ground being connected to the negative side of the sensing amplifier. Of course, it will be appreciated that the polarity of the connections to the sensing amplifier could be switched from this arrangement and this disclosure then would still remain applicable by merely switching all present references herein of downstroke to upstroke, and vice versa.